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Foraging Modes of Chinstrap Penguins: Contrasts Between Day and Night

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Foraging Modes of Chinstrap Penguins: Contrasts Between Day and Night MARINE ECOLOGY PROGRESS SERIES Vol. 165: 161-172,1998 Published May 7 Mar Ecol Prog Ser l Foraging modes of chinstrap penguins: contrasts between day and night John K. Jansen*,Peter L. Boveng, John L. Bengtson National Marine Mammal Laboratory, Alaska Fisheries Science Center. National Oceanic and Atmospheric Administration, 7600 Sand Point Way N.E., Building 4, Seattle. Washington 98115-0070, USA ABSTRACT: Penguins rely on vision to travel and hunt at sea. Vision in marine predators, particularly those hunting phototactic prey under a broad range of light intensities, must be better understood to realize how these species respond to changes in their environment. We studied the effects of daily cycles in light intensity on visual predators by examining the duration and timing of chinstrap pen- guins' PjrgosceUs antarctica foraging trips and the size, composition, and timing of their meals. We used radio telemetry and stomach-contents sampling to study adult penguins that were provisioning chicks during the summers of 1993 and 1994 at Seal Island, Antarctica. The penguins rarely initiated or ter- minated foraging trips at night, but otherwise varied the timing and duration of trips to sea. Cluster analyses using departure and arrival times revealed 5 distinct modes of foraging: 3 were strictly diur- nal (early, mid-, and late) and 2 were partly nocturnal (overnight and extended). Durations of diurnal trips (4 to 11 h) were shorter than overnight (13 to 14 h) and extended trips (18 to 22 h). Early and rnid- diurnal trips and extended trips were significantly shorter in 1993 than in 1994; late diurnal and overnight trip durations did not differ between years. Diurnal foraging was most common in 1993, whereas overnight foraging predominated in 1994. Shortened diurnal foraging in 1993 appears to have increased the frequency of diurnal foraging by allowing more parent birds to alternate diurnal trips within a single day and by reducing the incidence of birds extending diurnal foraging through the night. That penguins foraged more frequently by day when permitted by shorter trip durations (in 1993) suggests that they opted to forage diurnally whenever possible. Returning dlurnal and overnight foragers had greater than 99 and 74 % Antarctic krill Euphausia superba by weight in their stomachs, respectively However, overnight foragers also returned with significant amounts of highly digested remains of pelagic fish, suggesting birds were in offshore waters talung fish during the night. In con- trast, only 1 out of 40 diurnal foragers from both years combined had evidence of fish. Thus, the daily light cycle affected both the timing and duration of chinstrap penguin foraging as well as the type of prey consumed during trips to sea. KEY WORDS: Die1 activity patterns . Diet composition . Foraging trip duration Myctophid fish . Ecological monitoring . Pygoscelis antarctica . Euphausia superba INTRODUCTION tin 1990b). Although most seabirds occupy nesting colonies at night, some occasionally or regularly Empirical and experimental evidence points to vision remain at sea (albatrosses: Weimerskirch & Wilson as the primary sense used by birds to negotiate their 1992; storm-petrels: Grubb 1974; shearwaters: Brooke environment by day and night (reviewed by Martin 1990; penguins: Kooyman et al. 1992). Diving seabirds 1990a, b), even though olfactory and magnetic cues regularly experience less light than surface feeders have also proven important (Presti 1985, Verheyden & and there is little evidence that they possess unusual Jouventin 1994, Nevitt et al. 1995).The vast majority of visual capacities (Martin & Young 1984, but see Bow- the world's bird species are active primarily during the maker & Martin 1985).That visual acuity in submarine day, and less than 1 % are active entirely at night (Mar- hunters changes as a function of solar elevation and prey depth has important implications for understand- ing the constraints on foraging behavior in penguins. Antarctic penguins are particularly relevant to under- O Inter-Research 1998 Resale of full article not permitted Mar Ecol Prog Ser 165: 161-172. 1998 standing visual constraints because most occupy an ing their diet, we were able to compare chinstrap pen- environment characterized by broad ranges of daily guins foraging under different light regimes and deter- light intensities, as well as extreme shifts in daylength. mine whether there may be energetic consequences of Foraging activity in many specles of penguins has varying light intensity. As part of a long-term study of been l~nkedto the daily light cycle. The typical pattern chinstrap penguins and their prey, the goals here were is for most birds to be at sea midday and ashore at 2-fold: to understand the basic foraging patterns of a night (Adelie penguins Pygoscelis adeliae, Yeates marine predator in relation to a fundamental physical 1971; gentoo penguins P. papua and chinstrap pen- variable, light, and to build upon a framework from guins P. antarctica, Trivelpiece et al. 1986; African pen- which future studies will be better able to distinguish guins Spheniscus demersus, Wilson 1985; emperor predator behavior caused by changes in marine prey penguins Aptenodytes forsteri, Kirkwood & Robertson resources from that due to phylogenetic constraints, 1997; Humboldt penguins S. humboldti, Wilson & Wil- such as visual limitations. son 1990; Magellanic penguins S. magellanicus, Sco- laro & Suburo 1994; rockhopper penguins Eudyptes chrysocome, Wilson et al. 1997). Some species, how- METHODS ever, dive at night (macaroni penguins E. chrysolo- phus, Croxall et al. 1988; king penguins A. patagoni- We studied chinstrap penguins at Seal Island, South cus, Kooyman et al. 1992), despite recent evidence Shetland Islands, Antarctica (60"59'S, 55" 23' W; Fig. 1) suggesting that when foraging nocturnally, penguins during the austral summers of 1993 and 1994 at 2 have lower prey capture rates than when feeding dur- breeding colonies: North Cove, a colony of approxi- ing the day (Wilson et al. 1993, Piitz & Bost 1994, Wil- mately 900 nests located at the edge of a large inter- son 1995. Wilson & Wilson 1995). A recent model of tidal pool about 70 m from the open sea, and Colony aquiiiic visudi ieeding indicated that daily variations in 72, with approximately 400 nests located on a 300 m light intensity, and thus visual range, may be more long beach (Fig. 1, inset). important to predator feeding than typical variations in Foraging activity. The presence or absence of adult prey abundance (Aksnes & Giske 1993). chinstrap penguins provisioning chicks at North Cove Marked light-dependent rhythms in penguin forag- colony was recorded using radio telemetry from 9 to 22 ing activity (see Wilson et al. 1989, Williams & Rothery January 1993 and from 8 to 19 January 1994. The end 1990, Golombek et al. 1991) suggest that the timing of date in both years was determined by the beginning of trips to sea is an important factor in foraging success. the post-guard phase, the point at which parents leave The synchronization of a colony's foraging patterns in chicks unattended in the colony and are able to forage response to environmental cues may enhance prey independently. Radio transmitters (Advanced Teleme- searching and capture through communication about try Systems, Isanti, MN, USA; reference to trade name feeding conditions (Ward & Zahavi 1973, Brown 1986) does not imply endorsement by National Marine Fish- and group foraging, a widespread behavior in pen- eries Service, NOAA) were deployed on the departing guins (Ainley 1972, Broni 1985, Wilson et al. 1986a, adult from each of 80 nests (1993, n = 40; 1994, n = 40) Norman & Ward 1993). Despite evidence that changes after a nest relief had occurred so as to minimize dis- in light affect the timing and efficiency of penguin for- turbance of the mate on the nest. The foraging activity aging, remarkably few studies have shown more than of instrumented penguins was measured beginning 1 d gross trends in the activity of penguin rookeries in after all 40 penguins were fitted with transmitters; this relation to light cycles (Wilson et al. 1989, Williams & delay was an effort to reduce the effects that handling Rothery 1990). Nor has mu.ch research focused on the the birds and disturbing the colony may have had on potential constraints of variable light for visually-hunt- foraging behavior. Radio transmitters (1.35 cm diame- ing penguins whose daily foraging budget may ter, 6.8 cm length) were attached with epoxy and a include nocturnality at sea. plastic cable tie to feathers at the middle of each pen- In this study, we examined the timing and duration guin's back, posterior to the point of the bird's maxi- of foraging trips taken by chinstrap penguins at mum girth to minimize drag (Bannasch et al. 1994), colonies where adults are known to spend time at sea with the whip antenna trailing behind. The instru- overnight (Bengtson et al. 1993). Because penguins ments were wedge-shaped at the an.terior end, had a relying on vision may be less effective hunters at night, frontal cross-sectional area of 1.4 cm2 and a 28.5 cm we predicted that the birds could enhance foraging by antenna, and weighed 20 g. Attachment of this type of feeding diurnally whenever possible or by adopting transmitter (<l% of the bird's cross-sectional area) on alternative feeding tactics at night. During our study, chinstrap penguins at Seal Island had no measurable penguins were rearing small chicks and the sun was effect on duration of foraging (Croll et al. 1996).How- below the horizon for at least 6 h per night. By evaluat- ever small, any drag caused by these instruments Jansen et al.. Foraging modes of penguins 163 Fig. 1 Location of Seal Island within the Antarctic Pemn- suIa region. The dotted line indicates the 1000 m isobath. Inset shows the loca- tions of North Cove colony and Colony 72 in relation to other chinstrap pen- guin colonies on Seal Island would have energetic and possibly behavioral conse- which were always egested first.
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